The present invention relates to a mechanical sensor for vehicle seat belt systems and, in particular, to a mechanical sensor for operating a spring-type seat belt pretensioner.
The seat belt systems with which some vehicles are equipped have belt pretensioners for applying a preload to the belt to pull it tightly against the occupant's body and more securely restrain the occupant in the seat in the event of a collision. Some belt pretensioners are associated with the belt retractor and operate by rotating the retractor reel in the belt-winding direction to tighten the belt. Other pretensioners pull down on the buckle to which the seat belt is fastened. Still other types of pretensioners form a loop in the belt or wind a segment of the belt onto a spool at a point between the buckle and the retractor. All types of pretensioners require a power source, and it is known to use either a piston/cylinder operated by detonation of gunpowder or a mechanical spring.
When the power source is of a type that uses gunpowder, an electrical trigger is usually used to ignite the gunpowder. Such systems are costly to make and install. There are advantages to pretensioners based entirely on mechanical components, i.e. springs as power sources and mechanical sensors to cause the release of the spring.
When a spring is used as a power source, it must produce a large force and operate through a relatively long stroke in order to pretension the belt extremely rapidly and tighten it enough to provide the desired pretension. A mechanical sensor for releasing the trigger mechanism must likewise provide a high releasing force and do so very rapidly.
Mechanical sensors of various designs have been proposed; examples are found in Japanese Utility Model Laid-open Publications No. 57-90159 and No. 58-33060 (Japanese Utility Model Publication No. 63-35017) and Japanese Patent Laid-open Publications No. 58-206765, No. 1-164650 and No. 1-164651. The sensors disclosed in the aforementioned publications comprise inertia bodies that directly engage a trigger that holds the spring in the set condition and upon movement by an inertial force operate the trigger to release the actuator spring of the pretensioner. In order to obtain a sufficiently high output force to operate the trigger, the inertia bodies in previously known sensors must have very large masses, which requires that they be of large size and high weight. Moreover, because the sensor is incorporated into the pretensioner in these known devices, it is subject to vibrations and other movements of the pretensioner, which must be taken into account in determining a suitable location for the use of such sensors. For example, it may not be suitable to use such sensors when the pretensioner is installed on a vehicle seat.